Beam current regulator for electron beam machines

ABSTRACT

In an electron beam machine for welding or cuting a workpiece, the current produced by the electron gun is regulated by a beam regulating system to maintain a programmed value. A feedback voltage proportional to beam current is compared with a commanded voltage proportional to desired beam current, any difference therebetween in the form of an error voltage being used to vary the bias voltage of the electron gun and modify the beam current. A novel saturable reactor circuit permits the bias voltage which is at a high potential to be varied by a DC current which varies about ground level. A novel latch and hold circuit permits the beam current regulator to be bypassed when the electron gun filament is &#39;&#39;&#39;&#39;peaked&#39;&#39;&#39;&#39;. A novel current adder circuit permits the automatic programming of multi-level beam currents.

i iTRQ United mates r'atent Anderson BEAM CURRENT REGULATOR FOR ELECTRON BEAM MACHINES 3,547 O74 l2/l970 Hirschfeld ..250/492A Primary ExaminerHarold A. Dixon [75] Inventor: 2:11;? B. Anderson, South Windsor, Attorney Agent, or Firm DOnald F Bradley [73] Assignee: United Aircraft Corporation, East [57] ABSTRACT Hartford. COTln- In an electron beam machine for welding or cuting a [22] Filed: May 21, 1973 workpiece, the current produced by the electron gun is regulated by a beam regulating system to maintain a PP 362,152 programmed value. A feedback voltage proportional to beam current is compared with a commanded volt- 52 U S C 315/291 219/121 EB 250 492 age PI'OpOltlOIlHl IO desired beam CUI'Ifil'lt, any differ- 315/311 ence therebetween in the form of an error voltage 511 int. Cl G05f 1/00, HOSb 37/02 being used to vary the bias Voltage of the electron gun [58] Field of Search 219/121 EM, 12] and modify the beam current. A novel saturable reac- 492 A. 3 5/29 293 307 31 1 I01 ClI'CUlt permits the bias voltage which iS at a i potential to be varied by a DC current which varies [56] References Cited about ground level. A novel latch and hold circuit permits the beam current regulator to be bypassed when UNITED STA! E PA TENTS the electron gun filament is peaked". A novel cur- ?767327 10/1956 Hellerlme 250/409 rent adder circuit permits the automatic programming g g of multi-level beam currents. 3:435:187 3/1969 Sciaky 219/121 EB 4 Claims, 6 Drawing Figures H6 5125750 & i fi i f a r flflFFf/Q cuee /vr #7 TUE/4566 av/m I /6 fl/f ffi /F// 66 044702 @546 T0,? map/ 4y l' l l l I I 4 T 1 l l l l 3X ()7 l f r l /l/0/1/ 4 01/55? MP/Vffl7/0/t/ A 22 WWW/0E6? 1,4 76%? fi/A/ /4/1/0 H040 L C/FC'U/T T p C0/WM4/1/0 a fffaaflak BEAM CURRENT REGULATOR FOR ELECTRON BEAM MACHINES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electron beam machines, and particularly to apparatus for regulating and controlling the beam current. A closed loop feedback control compares actual beam current with a commanded beam current, and any error is used to vary the bias to the electron gun.

2. Description of the Prior Art Regulation of the high voltage supply in electron guns by means of a feedback system is commonly employed to maintain the voltage at a desired level. When a voltage regulator is employed, changes in the line voltage or load do not appreciably affect the high voltage accelerating potential. However, because the electron gun is commonly situated in an atmosphere of gas and metal vapors created during welding, cutting or other operations, the beam current may vary even though the accelerating potential is maintained constant. Any changes in the perveance of the gun will change the beam current, and as a result the total beam power will change. Variations in beam power will affect the welding or cutting depth or rate and cause undesir able variations in the workpiece.

The present invention provides a beam current regulator which will maintain a commanded beam current regardless of variations in electron gun perveance or other changes in operating characteristics.

In accordance with one aspect of the present invention, there is provided an improved beam current regulator for an electron gun which can be used on existing electron beam machines, is easy to adjust and which performs all logical functions and comparisons at essentially ground potential.

In accordance with another aspect of the present invention, a unique electron gun DC bias control circuit utilizing a saturable reactor is provided for versatile and improved operation of the regulator.

Another feature of the present invention is a novel current adder circuit which enables several sequential levels of beam current to be programmed without returning to the zero current level in between.

A further feature of the present invention is a unique latch and hold circuit which momentarily holds the electron gun bias constant to permit peaking of the electron gun filament.

SUMMARY OF THE INVENTION the DC error voltage is converted into a current. The

DC current is then supplied to a saturable reactor circuit as a control for the AC current fed to the bias electrode of the electron gun. Peaking of the filament may be performed by disconnecting the DC error voltage and applying a constant DC voltage to the current regulator circuit by means of a latch and hold circuit. A novel current adder circuit enables multiple level command voltages to be applied to the electron gun bias sequentially.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shown in block diagram form the beam current regulating'system of the present invention.

FIG. 2 shows in schematic form the saturable reactor and bias supply of FIG. 1.

FIG. 3 shows schematically the details of the latch and hold circuit of FIG. 1.

FIG. 4 shows in partial block diagram form a com mand voltage generator which may be used in conjunction with FIG. 1.

FIG. 5 is a graph of a representative current waveform produced by the command generator of FIG. 4.

FIG. 6 shows schematically the details of the current adder of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring specifically to FIG. 1, there is shown the beam current regulating system of this invention. A triode electron gun shown generally in schematic form at 10 may be of the type used in any well-known electron beam machine such as a welding machine. In the typical triode electron gun, the filament or cathode 12 is maintained at a high negative potential such as 50 kv or greater relative to an anode 14 which is generally grounded. A bias electrode as shown at 16 is maintained at a potential slightly more negative than the cathode l2, and variations in the bias potential such as provided by bias supply 18 connected to the bias electrode 16 will control the current produced by the electron gun 10. Electrons emitted from the filament 12 are accelerated toward the anode 14 and pass through an aperture in its center to impinge on a workpiece shown at 20. Conventional electron beam machines also con tain vacuum chambers, focusing lens, etc. which are not shown in the figure for purposes of clarity.

The beam current which impinges on the workpiece 20 may be sensed by any of several conventional techniques. If the workpiece 20 is grounded, a small resistor (not shown) connected between ground and the high voltage supply will produce a DC voltage drop proportional to beam current as the electrons which comprise the beam current complete the circuit and return to the high voltage supply via the ground connection. The production of the DC feedback voltage proportional to beam current together with any amplifiers required for gain matching are shown in block 22.

The DC feedback voltage proportional to beam current produced in block 22 is compared in a comparator 24 with a DC command voltage proportional to desired beam current generated in block 26. In its simplest form the command voltage may be a potentiometer connected to a source of voltage, the operator of the electron beam machine varying the wiper of the potentiometer in accordance with the desired beam current. A more complex command voltage supply in which various beam current programs may be produced will be described in connection with FIGS. 4 and 5.

If the beam current differs from that set by the machine operator by means of command voltage 26, a DC error voltage will be generated in comparator 24 which is proportional to the difference between the desired beam current and the actual beam current. The DC error voltage is fed through a noise filter 28 where the 60 cycle noise inherently present in the feedback signal is removed, and then through a compensation block 30 and a nonlinear gain block 32 where the gains and lags necessary to maintain regulation and stability in the total loop are provided. The nonlinear transconductance of the electron gun and the linear transfer function of the bias supply are combined into a single compensating nonlinear gain in block 32.

The modified and filtered DC error voltage is then fed into a buffer amplifier 34 where the DC voltage and impedance levels are matched to the input of a current regulator 36. The current regulator converts the DC voltage from buffer amplifier 34 into a DC current. During peaking of the electron gun filament the output from the nonlinear gain block 32 is disconnected from buffer amplifier 34 and the output from a latch and hold circuit 40 is applied to the buffer amplifier 34 through switches 38 as will be described in conjunction with FIG. 3.

The current regulator or driver stage 36 supplies a DC current proportional to the desireed beam current to a saturable reactor stage 42 to which the high volt age AC is supplied via line 44. The output from the reactor 42 is an AC voltage having an amplitude proportional to the desired beam current which is then fed to the bias supply 18 which in turn rectifies the AC voltage and provides a DC bias voltage to bias electrode 16.

The bias supply is a hybrid system which controls bias voltage by direct application of a high potential DC voltage together with an additional bias voltage which is a function of the beam current and which is applied in a negative feedback mode directly to the grid 16. The additional DC bias voltage is controlled by varying the applied amplitude of the AC voltage supplied via line 44 by controlling the impedance of the saturable reactor 42 in response to a low voltage DC current from the current regulator 36. Because the bias voltage is at a high potential, it is desirable to regulate it by a low voltage signal and to isolate the high voltage AC from the regulator 36. FIG. 2 shows in detail the novel saturable reactor circuit which provides the required isolation and enables control of a high voltage system by a low voltage DC current.

Control of the electron beam gun triode configuration at ground level without the use of mechanical devices was a major problem which has been overcome by the use of a hybrid bias supply. As described above a hybrid bias is one in which the high voltage DC is applied to both the filament and the bias electrode through current limiting resistors with an additional variable DC voltage being superimposed on the high voltage DC. This variable DC is provided from a variable AC voltage through an isolation transformer and rectifier-filter circuit. The amplitude ofthe variable AC voltage is controlled by a DC current from a current regulator stage.

Referring to FIG. 2, the AC input from source 44 is applied to the primary winding of transformer T The full secondary voltage from transformer T is fed through two saturable reactor stages S1 and S2 and then across the primary winding of isolation transformer T The AC output from the secondary winding of transformer T is fed to bias supply 18 where it is combined with the high voltage DC, rectified, filtered and fed to the bias electrode 16. A signal line 50 is connected directly to isolation transformer T through switch SW1. The saturable reactors S1 and S2 are fed by DC current from current regulator 36 to vary the impedance of the reactors and regulate the AC voltage across transformer T Two reactors are used in order to attain a symmetrical AC output and minimize the AC feedback upon the current regulator 36.

The AC voltage on line 50 with switch SW1 closed is selected to be sufficient to assure cutoff of the electron gun if a malfunction occurs in the regulator system, i.e., switch SW1 is a safety device to turn the beam off when the switch is closed. With the switch SW1 open and maximum current flowing from current regulator 36 through saturable reactors S1 and S2, the impedance of reactors S1 and S2 is at a minimum and the voltage across transformer T is at its maximum value sufficient to supply a bias voltage to bias electrode 16 to turn off the electron gun. With minimum or zero current flowing through reactors S1 and S2 from current regulator 36, the impedance of S1 and S2 is at its maximum and the voltage across transformer T is at a maximum whereby the bias voltage at bias electrode 16 will produce maximum current in the electron gun.

The reactors S1 and S2 may be operated in their linear region to provide easier stabilization of the beam current control. An increase in DC control current produced by current regulator 36 decreases the impedance of S1 and S2 and decreases the electron beam current, and a decrease in DC control current produced by regulator 36 increases the impedance of S1 and S2 and increases the beam current. The DC windings of reactors S1 and S2 are connected in AC opposition to cancel out any AC feedback, the secondary windings of S1 and S2 appearing as pure DC paths.

Connected to the secondary winding of isolation transformer T is a full wave diode bridge rectifier 52 which converts the AC from saturable reactor stage 42 into DC. A choke 54 and a capacitor 56 connected across the output of bridge rectifier 52 smooth the DC from the rectifier. A parasitic load resistor 58 is connected across the capacitor 56. The high potential DC voltage for the electron gun shown as -V and which may be -50 kv or more is connected to one terminal of the diode bridge rectifier 52 and to the electron gun filament 12. The electron gun grid 16 is also connected to the high potential DC voltage V through load resistor 58. The rectified DC current from rectifier 52 passes through resistor 58 in a direction to always maintain grid 16 at a negative potential relative to filament 12 by an amount determined by the voltage drop across resistor 58, normally to 2,000 volts. In other words, the grid 16 is maintained at the high potential DC voltage -V with the additional DC voltage produced by the rectified DC from rectifier 52 superimposed thereon, the additional voltage always maintaining the grid 16 more negative than the filament 12 by an amount determined and controlled by the DC current from current regulator 36.

Each control stage in the current regulator system except for the bias supply stage 18 is varied about ground potential with only the bias supply 18 being at a high potential.

FIG. 3 shows the details of the latch and hold circuit 40 which is inserted between the nonlinear gain block 32 and the buffer amplifier 34. When a beam current regulator system as shown in FIG. 1 is employed to maintain the electron gun current at a programmed value, it is difficult to peak the electron gun filament. As the filament heats up, the resulting emission current increases and the beam current likewise increases up to the point of space charge saturation. When a beam current regulating system is used, it is difficult to adjust the filament temperature to its optimum value since any variation in the filament temperature for a fixed bias voltage will cause a variation in beam current, and the regulating system will then adjust the bias voltage to maintain a constant beam current. Thus the attainment of peak operating heat at the filament is masked by the inherent operation of the regulating system and the operator has no assurance that the electron gun is operat ing in its optimum mode.

This problem is overcome by the latch and hold circuit of FIG. 3. The beam current error signal produced in comparator 24 of FIG. 1, which is indicative of the difference between the desired beam current and the actual beam current and which after being filtered, compensated and amplified appears as the output from block 32, is maintained at a constant value for a sufficient time so that the electron filament temperature may be adjusted to its optimum value without changes occurring in the bias voltage. In other words, the latch and hold circuit effectively disconnects the regulator system from the circuit and in its place maintains the bias voltage at a fixed value while the filament is peaked".

Referring to FIG. 3, the error signal voltage from nonlinear gain block 32 is fed through signal line 60 into latch and hold circuit 40 where the voltage appear ing on line 60 is continuously updated and stored when switch 38c is connected to the positive voltage terminal +V. With switch 38c connected to the +V terminal (the regulate mode), the error signal voltage from nonlinear gain block 32 also passes through line 62 and through switch 38a to the buffer amplifier 34, to ultimately regulate the electron beam current. At this time switch 38b is open.

When switch 38c is moved to the filament peaking mode which is at ground potential, switch 38a is opened thereby disconnecting the error signal voltage on line 62 from the buffer amplifier 34, and switch 38b is closed thereby connecting the stored voltage signal in the latch and hold circuit 40 to the buffer amplifier 34. The voltage stored in the latch and hold circuit 40 at this time will be the last voltage level which appeared on line 60. While in the filament peaking" mode, the output from the latch and hold circuit 40 slowly decays and can be controlled by a capacitor 64 connected across the output from the latch and hold circuit.

Thus, when switch 38c is in the regulate position, normal beam current regulation occurs. With switch 38c in the filament peaking position, a constant voltage equal to the last occurring error voltage is fed to the regulator allowing the filament to be peaked without any changes occurring in the error voltage. After peaking has been effected, switch 38c is returned to the regulate position and normal beam current regulation is resumed. Since the beam current may have been changed as a result of the peaking operation, the control loop should be permitted to stabilize before normal operation of the system is resumed.

FIG. 4 shows how the command voltage generator 26 of FIG. 1 may be implemented to permit the electron beam to be operated at several preprogrammed beam current levels in a sequential manner without returning to the zero current level in between. The beam programmer utilizes a logic network to program an integrator and a current adder at the proper times to produce various sequential voltages which cause the electron gun to produce the desired current levels.

Referring to FIG. 4, when the beam "on switch is closed, the logic control 72 is activated and transmits activating signals to integrator 74 and current adder 76 in the proper sequence to cause the current adder to produce the desired output or command voltage. Upon actuation of logic block 72, a signal is sent to integrator 74 via line 78 and the integrator 74 generates three reference voltages, ElR, E2R and E3R, which are proportional respectively to a first maximum current level, a second maximum current level (which may be identical with the first reference current level), and a final current level (which may be zero). The ElR, E2R and E3R voltages are fed to the current adder 76. Potentiometers 80 and 82 are adjusted by the machine operator or automatically to produce voltage signals respectively proportional to the desired first current level ElC and the desired second current level E2C, and the EIC and E2C signals are also fed to the current adder 76. Timing signals are fed to current adder 76 from logic network 72 via line 84 to signal the appropriate times for initiating the desired command voltages.

FIG. 5 shows in graph form the generation of a two level ramp command voltage with a return to Zero, but it is apparent that other waveforms of beam current are possible, and that step functions may be incorporated in the control. The voltages ElR and E2R are identical and are proportional to maximum beam current. Voltage ElC is proportional to about 50 percent of maximum beam current, while voltage E2C is proportional to about 80 percent of maximum beam current. Voltage E3R is a voltage which subtracts from the programmed beam currents and provides a return to zero beam current.

FIG. 6 shows the implementation of the command voltage programmer in the current adder 76 to produce the command voltage function shown in FIG. 5. At time t, logic network sends a signal via line 84 to the current adder 76, and in response thereto switch 86 is closed. The voltage ElC (which is shown to be about 50 percent of the voltage ElR) is fed via line 88 to summing junction 90 and voltage ElR is fed to multiplier 96. At this time all other inputs to summer 90 are zero, and the output Ec from summer 90 is fed to comparator 24 of FIG. 1. FIG. 5 shows that voltage level ElC is reached by means of a time delay or ramp from the zero voltage level, but the level ElC could be reached by a step function as will be obvious to those skilled in the art. The voltage ElC is also fed to multipler 92 via line 94, but no output is generated from multiplier 92 and 96 at this time because both E2C and E2R are still zero.

At time 1 a second signal is fed from logic network 72 to current adder 76 to close switch 98 and apply voltage E2C to multiplier 96. Voltage E2R is also applied to multiplier 92 at this time, and voltage E2C is fed to multiplier 100 via line 102. Voltage EZC is shown to be about 80 percent of voltage E2R. The output from multiplier 100 is zero at this time because voltage E3R is zero. As E2R and EZC increase to theirfinal values, the output from summer 90 increases to a level equal to E2C as explained below.

With switch 98 closed, multiplier 96 receives voltages ElR and E2C, and its output EA is E1R-E2C-K (K is a proportionality factor). Multiplier 92 receives voltages ElC and E2R, and its output EB is E1C-E2R-K. The voltage EB is subtracted from voltage EA in differential stage 104, and the output from the differential stage 104 is fed to summer 90 to be added to the EIC voltage fed to summer 90 via line 88.

The output from differential stage 104 is EAEB.

EA ElR-EZC-K and EB E1C-E2R'K,

therefore EA-EB ElRE2C-K El C-E2R'K.

Since by design ElR E2R,

EAEB E2R-K (EZC-El C) and if K is set equal to l/EZR, then EA-EB EZC-El C.

The output from summer 90 now is At time i switch 106 is closed by a signal from logic network 72 via line 84, and voltage E3R (with a polarity opposite that from ElR and E2R) is fed to multiplier 100. Voltage E2C is fed to multiplier 100 via line 102. If E3R is made unity, the output from multiplier 100 is E2C, which is then fed to summer 90 and subtracts from the EC voltage fed to comparator 24. EC then reduces to zero. E3R can, however, assume other values which would reduce Ec to some voltage other than zero and command a fixed current level.

Upon termination of the command voltage program, logic network 72 instructs current adder 76 to open switches 86, 98 and 106, and E will then become zero.

It is obvious that other combinations of command voltages may be programmed by the novel current adder.

While the present invention has been described with respect to its preferred embodiment, it will be apparent that changes may be made in its construction and operation without departing from the scope of the invention as hereinafter claimed.

I claim:

1. A beam current regulator for an electron gun having a filament and a bias electrode comprising means for sensing the electron gun beam current and producing a beam current signal indicative thereof,

means for generating a beam command signal indicative of desired electron gun beam current,

a comparator for comparing said beam current signal with said beam command signal to produce a beam current error signal variable as the difference therebetween,

first and second saturable reactors having input and output windings, the output windings of said reactors being connected in series and the input windings thereof being connected in series opposition,

a source of alternating voltage,

means for connecting said alternating voltage source to one end of said series connected saturable reactor output windings,

means for connecting said beam current error signal to the input windings of said saturable reactors to vary the impedance of said saturable reactors inversely with the magnitude of said beam current error signal,

output means connected to the pposite end of said series connected saturable reactor output windings for producing an ac output signal having an amplitude variable with the magnitude of said beam current error signal,

means for rectifying said ac output signal to produce therefrom a dc bias voltage,

and means for connecting said dc bias voltage to said electron gun bias electrode.

2. A beam current regulator as in claim 1 and including means for producing a high potential negative dc voltage,

means for connecting said high potential negative dc voltage to said electron gun filament,

means for combining said dc bias voltage with said high potential negative dc voltage so that said combined voltage is more negative than said high potential dc voltage by the amount of said dc bias voltage,

and means for connecting said combined voltage to said electron gun bias electrode.

3. A beam current regulator as in claim 1 wherein said means for producing a beam command signal comprises integrator means producing first and second voltage reference signals of equal magnitude,

means for selectively producing from said first and second voltage reference signals respectively first and second variable command voltage signals, and

means for sensing the electron gun beam current and producing a beam current singal indicative thereof,

means for producing a beam command signal indicative of desired electron gun beam current,

means comparing said beam current signal with said beam command signal to produce a beam current error signal variable as the difference therebetween,

means responsive to said error signal for generating a dc bias voltage,

means for connecting said dc bias voltage to said bias electrode,

a signal storage circuit,

nal from said bias voltage generating means and connecting in place thereof the value of the error signal stored in said signal storage circuit.

mg TES PATENT OFFICE CERTH ICAE OF CORRECTION Ne 3,838,313 Dated September 24, 1974 Inventoz w) Harry B. Anderson It is certified that error appears in the shove-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 7, "shown" should be --shows- Column 4, lines 23-24, "maximum" should be -minimum-- Column 7, line 35, both "EC" should be --Ec-- Column 8, claim 1, line 10, "pposite" should be --opposite-- Column 8, claim 4, line 55, "singal" should be ---signal-- Signed and sealed this 12th day of November 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. A beam current regulator for an electron gun having a filament and a bias electrode comprising means for sensing the electron gun beam current and producing a beam current signal indicative thereof, means for generating a beam command signal indicative of desired electron gun beam current, a comparator for comparing said beam current signal with said beam command signal to produce a beam current error signal variable as the difference therebetween, first and second saturable reactors having input and output windings, the output windings of said reactors being connected in series and the input windings thereof being connected in series opposition, a source of alternating voltage, means for connecting said alternating voltage source to one end of said series connected saturable reactor output windings, means for connecting said beam current error signal to the input windings of said saturable reactors to vary the impedance of said saturable reactors inversely with the magnitude of said beam current error signal, output means connected to the pposite end of said series connected saturable reactor output windings for producing an ac output signal having an amplitude variable with the magnitude of said beam current error signal, means for rectifying said ac output signal to produce therefrom a dc bias voltage, and means for connecting said dc bias voltage to said electron gun bias electrode.
 2. A beam current regulator as in claim 1 and including means for producing a high potential negative dc voltage, means for connecting said high potential negative dc voltage to said electron gun filament, means for combining said dc bias voltage with said high potential negative dc voltage so that said combined voltage is more negative than said high potential dc voltage by the amount of said dc bias voltage, and means for connecting said combined voltage to said electron gun bias electrode.
 3. A beam current regulator as in claim 1 wherein said means for producing a beam command signal comprises integrator means producing first and second voltage reference signals of equal magnitude, means for selectively producing from said first and second voltage reference signals respectively first and second variable command voltage signals, and current adder means including multiplier means for receiving said first and second voltage reference signals and said first and second command voltage signals, said current adder means producing a beam command signal which will cause said beam current to achieve a first level as a function of said first command voltage signal, and subsequently to achieve a second level as a function of said second command voltage signal.
 4. A beam current regulator for an electron gun, said gun having a filament and a bias electrode, comprising means for sensing the electron gun beam current and producing a beam current singal indicative thereof, means for producing a beam command signal indicative of desired electron gun beam current, means comparing said beam current signal with said beam command signal to produce a beam current error signal variable as the difference therebetween, means responsive to said error signal for generating a dc bias voltage, means for connecting said dc bias voltage to said bias electrode, a signal storage circuit, means connecting said error signal with said storage circuit to store therein the value of said error signal, and switching means for disconnectiNg said error signal from said bias voltage generating means and connecting in place thereof the value of the error signal stored in said signal storage circuit. 